Fuel cell end unit with integrated heat exchanger

ABSTRACT

An end unit for a fuel cell stack is described that contains an integrated heat exchanger for superheating the fuel gas before delivery to the stack. Heat is transferred from the hot cathode outlet stream to the cool fuel inlet stream in a space adjacent the stack&#39;s end plate. The end unit is designed as a hollow box forming the shell of the exchanger with the heat exchanger inside. The end unit has openings that allow fuel cell process gas to be taken directly from the stack without requiring piping or ductwork to be attached to thin manifolds. Separate chambers are provided for both the cathode outlet and anode outlet gas, thereby allowing all process connections to be made at one end of the stack. The end unit also features a current collection post that is separated from the end cell of the stack by a multitude of members which provide structural support for the end unit and act to more uniformly collect electrical current than would a single, large current post.

BACKGROUND OF THE INVENTION

[0001] This invention relates to fuel cells and, in particular, to endplates and heat exchangers for fuel cell systems. More specifically,this invention relates to highly integrated, compact heat exchangers foruse in superheating fuel gas for high temperature fuel cells.

[0002] A fuel cell is a device which directly converts chemical energystored in a fuel such as hydrogen or methane into electrical energy bymeans of an electrochemical reaction. This differs from traditionalelectric power generating methods which must first combust the fuel toproduce heat and then convert the heat into mechanical energy andfinally into electricity. The more direct conversion process employed bya fuel cell has significant advantages over traditional means in bothincreased efficiency and reduced pollutant emissions.

[0003] In general, a fuel cell, similar to a battery, includes anegative (anode) electrode and a positive (cathode) electrode separatedby an electrolyte which serves to conduct electrically charged ionsbetween them. In contrast to a battery, however, a fuel cell willcontinue to produce electric power as long as fuel and oxidant aresupplied to the anode and cathode, respectively. To achieve this, gasflow fields are provided adjacent to the anode and cathode through whichfuel and oxidant gas are supplied. In order to produce a useful powerlevel, a number of individual fuel cells must be stacked in series withan electrically conductive separator plate between each cell.

[0004] In a conventional fuel cell stack for stationary powerapplications, the active area of the fuel cells is large, typicallybetween {fraction (1/2)} and 1 m². In order to apply a reasonableinterface pressure on the cells, a large compressive load must beapplied to the cells through the end plates. As the end plates mustremain flat to insure intimate contact is maintained with the cells, theend plates are typically thick relative to their length and width. Thisthickness adds to the overall length of the fuel cell stack and size ofthe fuel cell power plant.

[0005] In addition, for high temperature fuel cell systems, a heatexchanger is required to heat the fuel gas to near the temperature ofthe stack prior to delivery to the stack. In one type of fuel cellsystem, this heat exchanger is placed external to the fuel cell stack aspart of the balance of the plant. This requires additional space toaccommodate the fairly thick insulation (2-3 inches) used to encase theheat exchanger. Also, in this type of system, process gas must be pipedto and from the heat exchanger, adding to both the size and cost of thesystem.

[0006] As described in U.S. Pat. No. 5,856,034, insulation for the heatexchanger can be eliminated by placing the heat exchanger inside thealready insulated fuel cell module enclosing the fuel cell stack.Specifically, the heat exchanger is placed upstream and adjacent thecathode inlet face of the stack, making it necessary to construct theexchanger large enough so as to completely cover the cathode inlet face.Also, in this system, due to the inherent non-uniform temperaturedistribution at the outlet of the heat exchanger, the stack inlettemperature distribution is also non-uniform. This condition isundesirable as non-uniform cathode inlet temperature not only creates apotential performance variation in the stack but also creates the riskof cell-to-cell wet seal leaks due to thermal expansion differences ofthe stack face.

[0007] U.S. Pat. No. 5,009,968 describes an end plate structure in whicha thin membrane is used to maintain good electrical contact with the endcells of the fuel cell stack. The thin membrane structure is notspecifically adapted to uniformly collect electrical current from thestack. U.S. Pat. No. 4,719,157 describes a thin end plate with multiplecurrent collecting terminals used to inhibit deformation of the plate.Again, this arrangement is not specifically adapted to provide uniformcollection of electrical current.

SUMMARY OF THE INVENTION

[0008] The present invention provides an end unit of a fuel cell stackhaving an assembly adapted to receive and convey gases in a heatexchange relationship, and/or to restrict electrical current flow fromthe fuel cell stack to a current collection post.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The above and other features and aspects of the present inventionwill become more apparent upon reading the following detaileddescription in conjunction with the accompanying drawings in which:

[0010]FIGS. 1 and 2 are isometric views of the end unit in accordancewith the principles of the present invention;

[0011]FIG. 3 is a cross-sectional top plan view of the end unit of FIG.1 taken along the line 3-3 of FIG. 2;

[0012]FIG. 4 is a front cross-sectional view of a fuel cell stackutilizing the end unit of FIG. 1;

[0013]FIG. 5 is a cross-sectional side view of a conventional fuel cellstack; and

[0014]FIG. 6 is a side cross-sectional view of the fuel cell stack ofFIG. 4.

DETAILED DESCRIPTION

[0015] Referring to FIGS. 1, 2 and 3, the illustrative embodiment of theinvention has an end unit 1 adapted to be attached to one end of a fuelcell stack. The end unit 1 houses an assembly 2 including first andsecond units 2A, 2B associated with the flow of a first and second gas,respectively, through the assembly and which act together as a heatexchanger. In the case shown, the first and second gases are fuel cellstack anode or fuel inlet gas and fuel cell stack cathode exhaust gas,respectively.

[0016] Particularly, the first unit 2A has an inlet 3 through which fuelgas passes (depicted by arrow 7) into an inlet chamber 18. Fuel gascollects in inlet chamber 18, flows in a direction 8 through a first setof tubes 19A and is delivered to a turn plenum 20. Fuel gas flows in adirection 9 through the turn plenum 20 and from the plenum 20 flows in adirection 10 through the tubes 19B. The gas is delivered by tubes 19B toan outlet chamber 21 (shown in FIGS. 2 and 3). Fuel gas exits the outletchamber 21 through an outlet pipe 4 in a direction indicated by arrow 11and, as described in further detail below with respect to FIG. 6, flowsthrough the fuel cell stack.

[0017] The second unit 2B forms an enclosure for the first unit 2A. Inthe illustrative embodiment, the second unit 2B has a first (or top)plate 14, opposing side walls 15A and 15B, front and back walls 25A and25B, and second (or bottom) plate 16 so as to create a generally hollowbox structure of the appropriate length and width to match the fuel cellstack and the appropriate depth so as to remain flat within a desiredtolerance upon compressive loading of the stack. As shown in FIGS. 1 and2, the first unit 2A is contained within the hollow interior of thesecond unit 2B. In addition, an inlet port 5 (shown in FIG. 1) and anoutlet port 6 (shown in FIG. 2) are formed in opposing side walls 15A,15B of the unit 2B. Also shown in the second unit 2B is a plurality ofmembers 17 extending between the first plate 14 and second plate 16 toprovide structural support for the second unit 2B. Members 17 will bedescribed in further detail below with respect to the current collectioncharacteristics of the end unit.

[0018] Fuel cell stack cathode gas enters the second unit 2B throughinlet port 5 (depicted by arrow 12 in FIG. 1) and flows in a directionsubstantially transverse to the plurality of tubes 19A, 19B. Asdescribed above with respect to the first unit 2A, fuel gas flows alongpaths 8 and 10 in the first and second sets of the multitude of tubes19A, 19B. Collectively, the tubes 19A, 19B have the required heattransfer surface area to adequately transfer heat from the hot cathodegas to the fuel gas, thereby raising the temperature of the fuel gas tothe desired temperature for delivery to the stack. The cathode outletgas exits the end plate through opening 6 (as shown by direction 13 inFIG. 2).

[0019] The tubes 19A, 19B of the first unit 2A are designed to bemechanically separated from the first (or top) plate 14 forming the endof the stack, second (bottom) plate 16 and side walls 15A, 15B of thesecond unit 2B. This configuration prevents both excessive stress on thejoints of the unit 2A and thermal distortions from affecting theflatness of the top and bottom plates 14, 16 of the second unit 2B.

[0020] Also depicted in FIGS. 1, 2 and 3 is a separate chamber 23 in thesecond unit 2B adapted to collect the anode outlet gas from the fuelcell stack by way of an anode outlet manifold (not shown). Fuel cellstack anode outlet gas is delivered to the chamber 23 through an inletopening 22 formed in a rear wall 25B of the second unit 2B and exits thechamber 23 through an outlet opening 24 formed in a side wall 15A. Withthe above configuration for the end unit, all gas connections (ducts,pipes and bellows) for delivering and removing process gases to and fromthe stack are made through the end unit 1 at one end of the stack.

[0021] The path of cathode gas flow through a fuel cell stack employingthe end unit 1 of the invention is shown in the fuel cell stackcross-sectional view of FIG. 4. First, cathode inlet gas enters the fuelcell stack 104 along a first face 104A of the stack in a directiondepicted by arrows 101. The cathode gas flows through the stack andexits the stack from a second stack face 104B opposite the first(cathode gas inlet) stack face 104A. Attached to the face 104B is acathode outlet gas manifold 106. Cathode outlet or exhaust gas iscollected in the cathode outlet gas manifold 106 and flows through thecathode outlet gas manifold 106 in a direction shown by arrow 102. Thecathode outlet gas manifold 106 delivers cathode outlet gas to the endunit 1 through opening 5. Cathode outlet gas then flows through the endunit in a direction represented by arrow 103 and as described above withrespect to FIGS. 1-3, and exits the end unit through opening 6. In thisconfiguration, as previously stated, heat is transferred from the fuelcell stack cathode exhaust gas to the anode inlet gas by heat exchangein the end unit 1.

[0022] In a conventional system, as shown in FIG. 5, heat is taken froman inlet stream (depicted by arrows 201) of fuel cell stack cathode gas,requiring an assembly 207 for heat exchange between cathode and anodegases to be disposed along an entire stack face. After flowing throughthe heat exchanger 207, cathode inlet gas flows through the stack 204and exits the stack into a cathode outlet gas manifold 206. By providingheat exchange in the end unit attached to the end of a fuel cell stackin accord with the invention, rather than along an entire stack face asin the conventional structure of FIG. 5, large space requirements,non-uniformity in stack inlet temperature distribution and the risk ofcell-to-cell wet seal leaks, as discussed above, are obviated.

[0023]FIG. 6 shows the path of anode or fuel gas flow through a fuelcell stack employing the end unit 1 of the invention. Fuel gas entersthe end unit 1 at inlet 309, fills the inlet plenum 18 (not shown inFIG. 6) and flows through the tubes 19A, turn plenum 20 (not shown inFIG. 6), and tubes 19B in a substantially U-shaped path as depicted byarrows 310. As described above, anode gas is superheated by the transferof heat from cathode outlet gas flowing transverse to the tubes 19A,19B. Next, the heated anode gas exits the tubes 19B and flows fromoutlet plenum 21 (not shown in FIG. 6) of the end unit 1 into a fuelheader 308 in a direction depicted by arrow 311. The fuel header 308 isdisposed within a fuel gas inlet manifold 307 and extends along thelength of the manifold 307. The fuel gas header 308 and manifold 307permit the heated fuel gas to exit the header and manifold at pointsalong the length of the manifold and flow into the fuel cell stack in adirection depicted by arrows 312. The flow of fuel gas through the fuelcell stack 104 as shown in FIG. 6 is in a direction 312 perpendicular tothe direction of the flow of cathode gas through the fuel cell stack,but the anode and cathode gas flow paths do not intersect. After flowingthrough the stack 104, the fuel gas enters an anode outlet gas manifold306 and flows in a direction depicted by arrow 313. The anode outlet gasmanifold 306 then delivers the gas to the anode outlet gas chamber 23 ofthe end unit 1 as it flows in a direction shown by arrow 314. In theanode outlet gas chamber 23 of the end unit 1, the anode outlet streamis collected by the necessary ductwork and piping to be delivered to thebalance of the fuel cell power plant.

[0024] With the end unit 1 of the invention, any fuel gas leaks that maydevelop over the life of the unit are immediately swept away from thestack by the cathode outlet gas. This is unlike the case of a heatexchanger placed upstream of the stack, in which a leak must first passthrough the stack and cathode outlet manifold before leaving the fuelcell module. The risk of a build-up of the mixture of gases within thefuel cell module is reduced.

[0025] Turning back to FIG. 4, current collection posts 105, 107 aredisposed at the positive and negative ends of the fuel cell stack. Thecurrent collection post 107 at the positive end of the stack is spacedfrom the first plate 14 by a plurality of members 17 (also shown inFIGS. 1-3). In the case shown, the members 17 are formed fromelectrically conductive material and are shaped as cylindrical columns.As shown in detail in FIGS. 1-3, the members 17 extend between the firstplate 14 and second plate 16 and are disposed at uniformly spacedintervals among the first and second sets of said tubes 19A, 19B and inthe anode outlet gas chamber 23. In the illustrative embodiment, thefirst plate 14 of the end unit 1 is in electrical contact with thestack, and the second plate 16 (see FIG. 1) is in electrical contactwith the current collection post 107. The members 17 thus provide anelectrical connection between the first and second plates 14, 16 of theend unit and additionally provide structural support to the end unit,distributing mechanical and thermal stresses in the end unit thatdevelop during operation of the stack.

[0026]FIG. 5 shows a cross-sectional side view of a conventional fuelcell stack. As shown in FIG. 5, current collection posts 205 at each endof the stack are disposed adjacent to the positive and negative ends ofthe stack and collect current directly from the stack. In the presentinvention shown in FIG. 4, the separation of the current collection post107 from the first plate 14 of the end unit 1 by the plurality ofmembers 17 is advantageous in that the members 17 act to restrictelectrical current flow slightly, allowing more uniform currentcollection from the stack through the uniformly spaced members 17.

[0027] In all cases it is understood that the above-described apparatus,method and arrangements are merely illustrative of the many possiblespecific embodiments that represent applications of the presentinvention. Numerous and varied other arrangements can be readily devisedin accordance with the principles of the present invention withoutdeparting from the spirit and the scope of the invention. For example,while shown in FIG. 4 at the positive end of the stack, the end unit maybe disposed at either the positive or negative ends of the fuel cellstack. Also, designs using a plate fin, compact heat exchanger, couldalso be configured.

What is claimed is:
 1. A fuel cell stack end unit, comprising: anassembly adapted to receive first and second gases, to convey said firstand second gases in heat exchange relationship and to output said gasesafter heat is transferred from one gas to another.
 2. A fuel cell stackend unit according to claim 1, wherein said assembly comprises: a firstunit for defining an enclosed gas flow path for said first gas; and asecond unit enclosing said first unit and defining a gas flow path forsaid second gas intersecting said first unit, wherein said first andsecond units act as a heat exchanger adapted to transfer heat from saidsecond gas to said first gas.
 3. A fuel cell stack end unit according toclaim 2, wherein said first unit includes a plurality of tubes forreceiving and carrying said first gas and said second unit comprises anenclosure in which said plurality of tubes are disposed.
 4. A fuel cellstack end unit according to claim 3, wherein said plurality of tubes hasa surface area adapted to transfer heat from said second gas flowing insaid second unit to said first gas flowing in said tubes.
 5. A fuel cellstack end unit according to claim 4, wherein said second unit is in theform of a hollow box forming said enclosure, said hollow box definingexterior walls of said heat exchanger.
 6. A fuel cell stack end unitaccording to claim 5, further comprising a third unit disposed withinsaid hollow box and adapted to receive a third gas.
 7. A fuel cell stackend unit according to claim 6, wherein said hollow box has first andsecond opposing exterior walls and said second unit further comprises aninlet port disposed in said first exterior wall of said hollow box andan outlet port disposed in said second exterior wall of said hollow box.8. A fuel cell stack end unit according to claim 7, wherein said secondgas flows into said second unit through said inlet port and flows out ofsaid second unit through said outlet port, and wherein said second gasflows through said second unit in a direction transverse to thedirection of said plurality of tubes in said first unit.
 9. A fuel cellstack end unit according to claim 8, wherein said first gas is fuel cellstack anode inlet gas and said second gas is fuel cell stack cathodeoutlet gas.
 10. A fuel cell stack end unit according to claim 9, whereinsaid fuel cell cathode outlet gas is delivered from said fuel cell stackto said inlet port of said second unit by a cathode outlet gas manifolddisposed on a side of said fuel cell stack.
 11. A fuel cell stack endunit according to claim 4, wherein said first gas is fuel cell stackanode inlet gas.
 12. A fuel cell stack end unit according to claim 11,wherein said second gas is fuel cell stack cathode outlet gas.
 13. Afuel cell stack end unit according to claim 12, further comprising athird unit adapted to receive a third gas from said fuel cell stack. 14.A fuel cell stack end unit according to claim 13, wherein said thirdunit is disposed within said second unit and wherein said third gas isfuel cell anode outlet gas.
 15. A fuel cell stack end unit according toclaim 14, wherein all connections for delivering and removing processgases to and from said fuel cell stack are disposed in said end unit.16. A fuel cell stack end unit according to claim 14, wherein said firstunit further comprises an inlet chamber and an outlet chamber, saidoutlet chamber being connected to said inlet chamber by said pluralityof tubes.
 17. A fuel cell stack end unit according to claim 16, whereinsaid first unit further comprises: a turn plenum, said turn plenum beingconnected by a first set of said plurality of tubes to said inletchamber and being connected by a second set of said plurality of tubesto said outlet chamber.
 18. A fuel cell stack end unit according toclaim 17, wherein said fuel cell anode inlet gas flows into said inletchamber, through said first set of said plurality of tubes, said turnplenum, and said second set of said plurality of tubes to said outletchamber in a substantially U-shaped path.
 19. A fuel cell stack end unitaccording to claim 17, wherein said outlet chamber of said first unitcommunicates said fuel cell anode inlet gas to an anode inlet gasmanifold disposed on a side of and providing anode gas to said fuel cellstack.
 20. A fuel cell stack end unit according to claim 19, whereinsaid fuel cell stack anode outlet gas is communicated from said fuelcell stack to said third unit of said stack end unit by means of ananode outlet gas manifold disposed on a side of said fuel cell stack.21. A fuel cell stack end unit according to claim 18, wherein saidsecond unit further comprises an inlet port formed on one side of saidenclosure and an outlet port formed on a side of said enclosure oppositesaid one side.
 22. A fuel cell stack end unit according to claim 21,wherein fuel cell stack cathode outlet gas enters said second unitthrough said inlet port and exits said second unit through said outletport, said fuel cell cathode outlet gas flowing through said second unitin a direction substantially transverse to the direction of said firstand second sets of said plurality of tubes in said first unit.
 23. Afuel cell stack comprising a plurality of fuel cells and a fuel cellstack end unit disposed at each end of the stack, one of said fuel cellstack end units including an assembly adapted to receive first andsecond gases, to convey said first and second gases in heat exchangerelationship and to output said gases after heat is transferred from onegas to another.
 24. A fuel cell stack according to claim 23, whereinsaid heat exchanger is adapted to superheat fuel gas before delivery tosaid stack.
 25. A fuel cell stack according to claim 23, wherein saidassembly comprises: a first unit for defining an enclosed gas flow pathfor said first gas; and a second unit enclosing said first unit anddefining a gas flow path for said second gas intersecting said firstunit, wherein said first and second units act as a heat exchangeradapted to transfer heat from said second gas to said first gas.
 26. Afuel cell stack according to claim 25, wherein said first unit includesa plurality of tubes for receiving and carrying said first gas and saidsecond unit comprises an enclosure in which said plurality of tubes aredisposed.
 27. A fuel cell stack according to claim 26, wherein saidplurality of tubes has a surface area adapted to transfer heat from saidsecond gas flowing in said second unit to said first gas flowing in saidplurality of tubes.
 28. A fuel cell stack according to claim 27, whereinsaid fuel cell end unit is in the form of a hollow box forming saidenclosure, said hollow box defining exterior walls of said heatexchanger.
 29. A fuel cell stack according to claim 28, furthercomprising a third unit disposed within said hollow box, said third unitbeing adapted to receive a third gas.
 30. A fuel cell stack according toclaim 27, wherein said first gas is fuel cell stack anode inlet gas. 31.A fuel cell stack according to claim 30, wherein said second gas is fuelcell stack cathode outlet gas.
 32. A fuel cell stack according to claim31, wherein said assembly further comprises a third unit adapted toreceive a third gas from said fuel cell stack.
 33. A fuel cell stackaccording to claim 32, wherein said third unit is disposed within saidsecond unit and wherein said third gas is fuel cell anode outlet gas.34. A fuel cell stack according to claim 32, wherein all connections fordelivering and removing process gases to and from said fuel cell stackare disposed in said assembly.
 35. A fuel cell stack according to claim34, wherein said first unit further comprises an inlet chamber and anoutlet chamber, said outlet chamber being connected to said inletchamber by said plurality of tubes.
 36. A fuel cell stack according toclaim 35, wherein said first unit further comprises: a turn plenum, saidturn plenum being connected by a first set of said plurality of tubes tosaid inlet chamber and being connected by a second set of said pluralityof tubes to said outlet chamber.
 37. A fuel cell stack according toclaim 36, wherein said fuel cell anode inlet gas flows from said inletchamber through said first set of said plurality of tubes, said turnplenum, and said second set of said plurality of tubes, to said outletchamber in a U-shaped path.
 38. A fuel cell stack according to claim 36,further comprising: an anode gas inlet manifold disposed on one side ofsaid stack, said anode gas inlet manifold receiving said fuel cell anodeinlet gas from said outlet chamber of said first unit and communicatingsaid fuel cell anode inlet gas to said fuel cell stack.
 39. A fuel cellstack according to claim 38, further comprising: an anode gas outletmanifold disposed on a side of said stack opposite said anode gas inletmanifold, said anode gas outlet manifold communicating said fuel cellanode outlet gas from said fuel cell stack to said third unit.
 40. Afuel cell stack according to claim 39, wherein said second unit furthercomprises an inlet port formed on one side of said enclosure and anoutlet port formed on a side of said enclosure opposite said one side.41. A fuel cell stack according to claim 40, wherein said fuel cellcathode outlet gas: enters said second unit through said inlet port;flows through said second unit in a direction substantially transverseto the direction of said first and second sets of said plurality oftubes; and exits said second unit through said outlet port.
 42. A fuelcell stack according to claim 41, further comprising a fuel cell cathodeoutlet gas manifold disposed on one side of said stack, said cathode gasoutlet manifold delivering said fuel cell cathode outlet gas from saidfuel cell stack to said inlet port of said second unit.
 43. An end unitfor attachment between one end of a fuel cell stack and a currentcollection post, said end unit comprising a structure adapted torestrict electrical current flow from said fuel cell stack to saidcurrent collection post.
 44. An end unit according to claim 43, furthercomprising a first surface in electrical contact with said fuel cellstack and a second surface in electrical contact with said currentcollection post, said structure being in electrical contact with saidfirst and second surfaces.
 45. An end unit according to claim 44,wherein said structure comprises a plurality of members, said membersconnecting said first and second surfaces of said end unit.
 46. An endunit according to claim 45, wherein said members of said structure areelectrically conductive.
 47. An end unit according to claim 46, whereinsaid members are disposed at uniformly spaced intervals between saidfirst and second surfaces.
 48. An end unit according to claim 47,wherein each of said members is a cylindrical post.
 49. An end unit forattachment to a fuel cell stack, said end unit separating a currentcollection post from the fuel cell stack, said end unit comprising astructure adapted to restrict electrical current flow from said fuelcell stack to said current collection post; wherein said structure isfurther adapted to receive first and second gases, to convey said firstand second gases in heat exchange relationship and to output said gasesafter heat is transferred from one gas to another.
 50. A fuel cell stackcomprising: a plurality of fuel cells; a current collection postdisposed at one end of said fuel cell stack; and an end unit forattachment to one end of said plurality of fuel cells and separatingsaid current collection post from said plurality of fuel cells, said endunit comprising a structure adapted to restrict electrical current flowfrom said plurality of fuel cells to said current collection post,wherein said structure is further adapted to receive first and secondgases, to convey said first and second gases in heat exchangerelationship and to output said gases after heat is transferred from onegas to another.
 51. A fuel cell stack comprising: a plurality of fuelcells; a current collection post disposed at one end of said fuel cellstack; and an end unit for attachment to one end of said plurality offuel cells and separating said current collection post from saidplurality of fuel cells, said end unit comprising a structure adapted torestrict electrical current flow from said plurality of fuel cells tosaid current collection post.